## Monday, August 02, 2021

### Metallic water!

What does it take to have a material behave as a metal, from the physicist's perspective?  I've written about this before (wow, I've been blogging for a long time).  Fundamentally, there have to be "gapless" charge-carrying excitations, so that the application of even a tiny electric field allows those charge carriers to transition into states with (barely) higher kinetic energies and momenta.

 Top: a droplet of NaK alloy.  Bottom: That droplet coated with adsorbed water that has become a metal. From here.
In conventional band insulators, the electronic states are filled right up to the brim in an energy band.  Apply an electric field, and an electron has no states available into which it can go without somehow grabbing enough energy to make it all the way to the bottom of the next (conduction) band.  Since that band gap can be large (5.5 eV for diamond, 8.5 eV for NaCl), no current flows, and you have an insulator.

This is, broadly speaking, the situation in liquid water. (Even though it's a liquid, the basic concept of bands of energy levels is still helpful, though of course there are no Bloch waves as in crystalline solids.)  According to calculations and experiments, the band gap in ordinary water is about 7 eV.  You can dissolve ions in water and have those carry a current - that's the whole deal with electrolytes - but ordinarily water is not a conductor based on electrons.  It is possible to inject some electrons into water, and these end up "hydrated" or "solvated" thanks to interactions with the surrounding polar water molecules and the hydronium and hydroxyl ions floating around, but historically this does not result in a metal.  To achieve metallicity, you'd have to inject or borrow so many electrons that they could get up into that next band.

This paper from late last week seems to have done just that.  A few molecular layers of water adsorbed on the outside of a droplet of liquid sodium-potassium metal apparently ends up taking in enough electrons ($\sim 5 \times 10^{21}$ per cc) to become metallic, as detected through optical measurements of its conductivity (including a plasmon resonance).   It's rather transient, since chemistry continues and the whole thing oxidizes, but the result is quite neat!

Anonymous said...

I love this sort of science, built upon a long tradition of playing *cough* experimenting. In my experience, it is stuff like this which opens new directions in research.

I really worry that this would be difficult to do in the USA nowadays. During my time in the National Lab system, we had to drop more cool ideas than I can remember due to the vagaries of the funding system. I also note that the authors used a 'local' synchrotron source (BESSY-II), which has a much more flexible approach to experiments than many of the bigger facilities. Generally speaking scientists come up with cool stuff when they have time and (some) freedom from oversight.

Anonymous said...

@Anonymous, to be fair, the equipment used to prove the concept worked before applying for beamtime was quite literally funded by viewers on YouTube, not BESSY. The first author is a well-known YouTuber by the name of Thunderf00t, and the vacuum chamber used to first show the effect (before going to a synchrotron) was funded by his viewers.

See Phil's video explanation here: https://www.youtube.com/watch?v=Vdz18ibX7rE

I am have doubts over whether local vs National Lab synchrotron played a big role here (though maybe in how readily beamtime was allocated). Seems like more serendipity and curiosity than anything else.

Anonymous said...

Quoting the acknowledgments of the paper itself "P.E.M. acknowledges support from the viewers of his YouTube popular science channel"... :)!